This invention pertains to a system for determining the location of wireless devices.
A number of systems have been implemented or proposed that provide for the determination of location information using various techniques. The Global Positioning System (GPS) is an example of a known system that employs satellites to transmit location determining information. A GPS receiver uses these satellite transmissions as reference data to determine its position. A disadvantage of GPS is that in some locations, such as urban canyons or within buildings, signals from GPS satellites may be occluded.
For applications that require location determination inside an office building or the like, one approach is to arrange a group of receivers at fixed locations within the building, and to attach a transmitter to an object that is to be located. The fixed receivers are connected to a central computer that determines the location of an object based on signals received from the associated transmitter through one or more of the group of receivers. Unfortunately this type of system requires a substantial amount of installed infrastructure, including the fixed receivers, and wired connections between the receivers and base stations. Moreover, these systems do not provide precise determination of location.
The strength of a received signal can in principle be used to determine the distance between a transmitter and a receiver. Another technique commonly used in determining location information is known as time difference of arrival (TDOA). It is known that these and other techniques used to determine relative location information are subject to inaccuracies caused by various factors, including channel attenuation, interference, and the like. It would be desirable to improve the accuracy of systems for determining location. Therefore, an improved method of determining relative location for wireless devices is needed.